Processes for the production of methanol by catalytic conversion of synthesis gas containing hydrogen and carbon oxides have long since been known to those skilled in the art. For example in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, Chapter “Methanol”, Sub-chapter 5.2 “Synthesis”, various basic processes for the production of methanol are described.
A more advanced, two-stage process for the production of methanol is known for example from EP 0 790 226 B1. Methanol accordingly is produced in a cyclic process in which a mixture of fresh and partly reacted synthesis gas first is supplied to a water-cooled reactor and then to a gas-cooled reactor, in each of which the synthesis gas is converted to methanol on a copper-based catalyst. The methanol produced in the process is separated from the synthesis gas to be recirculated, which then is countercurrently guided through the gas-cooled reactor as coolant and preheated to a temperature of 220 to 280° C., before it is introduced into the first synthesis reactor. A part of the synthesis gas to be recirculated is removed from the process as purge stream (so-called purge), which is small as compared to the gas inventory present in the system, in order to prevent that inert components, impurities or by-products accumulate within the synthesis gas circuit. This measure also is taught in the unexamined German Patent Application DE 2934332 A1 and in the European Patent Application EP 1016643 A1.
This process however requires that the stoichiometric number (R) of the synthesis gas used, defined by the formula:R=([H2]−[CO2])/([CO]+[CO2]),with [x]=concentration of the component x, is at least 2, that the synthesis gas used hence contains enough hydrogen with regard to the production of methanol. Synthesis gases with hydrogen deficiency, on the other hand, can be obtained from reforming processes which include a stage of partial oxidation, such as the autothermal reformation (ATR). In such a case the hydrogen is consumed in the methanol synthesis reaction, while a major part of the carbon oxides is left unreacted, which leads to a composition in the synthesis loop which has a high content of carbon oxides, but is poor in hydrogen, i.e. has a hydrogen deficit or a hydrogen deficiency. This has various consequences, which include the fact that the required catalyst volume is high and that the fraction of by-products (in particular higher alcohols and ketones) is distinctly higher than normal.
It is known per se to supply hydrogen from other sources to the synthesis gas deficient in H2, in order to bring the stoichiometric number into the optimal range of 2 or higher.
As hydrogen source the purge stream can be used, among other things, which still has a content of non-converted hydrogen that can be separated by means of a hydrogen recovery unit. The typical hydrogen concentration in the purge stream is about 70 vol-%. Another alternative consists in recovering hydrogen from a side stream of the fresh synthesis gas, which also is referred to as make-up gas (MUG), and to feed this hydrogen back into the synthesis gas. The laid-open US patent application US 2009/0018220 A1 teaches a methanol synthesis process in which hydrogen is obtained from at least a part of said purge gas and a part of said make-up gas, wherein the recovered hydrogen is introduced into the synthesis gas mixture. The hydrogen recovery units used include the pressure swing adsorption (PSA) or—as an alternative—the H2 separation by means of a membrane.
The disadvantage of this arrangement, however, consists in that within the hydrogen recovery unit a part of the hydrogen gets lost, before at all reaching the synthesis loop. In addition, when the synthesis gas make-up gas stream already contains a hydrogen deficit, the same only is increased by additional separation of H2 from a make-up gas side stream. Due to the separation of H2 from the make-up gas side stream, a waste stream containing CO and CO2 then is obtained in the hydrogen recovery unit, wherein the carbon oxides of the methanol synthesis contained therein get lost.